Abstract The first metatarsophalangeal joint (MTPJ1) is one of the sites most commonly affected with osteoarthritis (OA). Worldwide, more than 1 in 5 people are estimated to have pain and functional deficits from MTPJ1 OA. In contrast to the widely performed joint replacement arthroplasties for degenerative OA at the hip and knee joints, the two most common surgical treatments for MTPJ1 osteoarthritis are cheilectomy and, for more severe cases, arthrodesis. Neither of these approaches have high rates of patient satisfaction. To date, various designs for MTPJ1 arthroplasty have been proposed, but none have been particularly successful. This is partly because of the relatively small amount of bone in the metatarsal head and proximal phalanx, making it difficult to achieve adequate fixation of the prosthetic components, leading to loosening of the implant over time. Hemi- caps are also available for both the metatarsal head and the phalangeal base and involve the removal of less bone, however failure rates remain high. Development of new implants that can address these problems is limited by the sparseness of the literature describing the mechanical environment of the MTPJ1, and there are no standardized open source models of the MTPJ1 available to the research community. Similarly, there is very little detailed information in the literature on the required 3D movement of the MTPJ1 during locomotor activities and what data is available is either from cadaver studies or from surface markers attached to the feet of walking subjects. It is likely that these approaches do not adequately capture the range or complexity of MTPJ1 movement due to unrealistic function and soft tissue artifacts. In this proposal we intend to fill these voids and to generate design criteria for MTPJ1 arthroplasty by: 1) building detailed, parametric models of the MTPJ1 using OpenSim and FEBio; 2) measuring the 3D motion of the first proximal phalanx and the 1st metatarsal bones in different footwear conditions during locomotion using biplane fluoroscopy. In the final phase of the project we will combine information from these two approaches and develop prototypes for new prostheses designed to overcome some of the limitations of previous devices. Specifically, we will explore methods to utilize screw fixation, similar to locking plates. Prototypes of these designs will be 3D printed in stainless steel and tested to assess feasibility and integrity in loading experiments with cadaver feet. We believe that the proposed work has the potential to reinvigorate the innovative study of MTPJ1 replacement, which is, at present, primarily driven by ideas not by data.